The invention relates to the production of polyalkenes of wide molecular weight distribution.
The invention is directed to a process for polymerizing .alpha.-alkenes of 2 to 8 carbon atoms, and in particular to the polymerization of ethylene and propylene. The polymerization process of the invention contemplates the polymerization of .alpha.-alkenes, optionally with minor amounts of at least one other .alpha.-alkene of 2 to 10 carbon atoms.
The polymerization is undertaken in the presence of a catalyst comprising at least one compound of a transition metal selected from the group consisting of Groups 4 to 6 or 8 of the Periodic Table. Practically, an organometallic compound of a metal from Groups 1 to 3 of the Periodic Table is admixed with the compound of the transition metal. The catalyst may be of the type described in U.S. Pat. Nos. 3,257,332; 3,377,332 and 3,574,138, all of which are hereby incorporated by reference herein. However, the exact nature of the catalyst composition used to undertake the polymerization of the invention is not critical.
The polymerization process may be undertaken at pressures of up to 100 kg/cm.sup.2 in a liquid solvent to produce a polymer with ASTM D-1238 melt index of below 100 and with a wide molecular weight distribution. These results of the process are realized by undertaking the polymerization in at least two reactors which employ similar catalyst systems and under polymerization conditions in one of the reactors which effects the formation of alkene polymer of high molecular weight, compared to the molecular weight of the polymer produced in the other one of said at least two reactors. Concomitantly, the polymerization conditions in said other reactor effects the formation of a polymer with a low molecular weight compared to that polymer of high molecular weight.
Polyalkenes of wide molecular weight distribution are particularly desirable for applications of polyalkenes, and in particular in applications of polyethylene, such as, e.g., extruding bottles, cables, tubes and the like, and in blow-molding. Polyethylenes, characterized by wide molecular weight distribution, when used in the aforementioned processes, can be used to produce articles of better surface gloss, while surface defects, including melt fracture are substantially obviated. Thus, in processing polyethylenes of similar melt indices, that polyethylene with the widest molecular-weight distribution will yield articles with best surface gloss, with fewer surface defects, such as that described above.
Low molecular weight, i.e. and high melt index, polyalkenes are characterized by comparatively high flow rates in extrusion processes, with relatively infrequent occurrence of melt fracture. However, low molecular weight polyalkenes produce articles of mechanical structure characterized by relatively poor tensile strength, toughness, and stresscrack resistance, compared to high molecular weight polyalkenes.
Polyalkenes of high molecular weight, particularly high molecular weight polyethylene and polypropylene, are preferred for use in the manufacture of articles by extrusion or blow-molding. The use of polyalkenes of relatively high molecular weight and wide molecular-weight distribution obviates the problems of poor flow, lack of surface gloss and surface defects. The desirable molecular-weight distributions and molecular-weight values are elucidated in Modern Plastics, Pages 109-112 (December 1957) and in Technical Papers, Volume 7 of the 17th Annual Technical Conference of the Society of Plastics Engineers 28-1, Pages 1-4, which are incorporated herein by reference.
Many processes for the preparation of polyalkenes, in particular polyethylene, of wide molecular weight distribution have been described.
Belgian patent specification No. 551,931 suggests mixing polyethylene of high molecular weight with polyethylene of low molecular weight to obtain polyethylene which is characterized by the desired properties. British patent specification No. 1,233,599 proposes a similar process. The disadvantages of the products produced by such mechanical mixtures is that the products are characterized by a lack of homogeneity which is especially manifest and unacceptable in the manufacture of thin-walled articles, such as films, bottles, and the like.
Further attempts to produce high molecular weight polyalkenes of wide molecular weight distribution have been disclosed to reside in the selection of particular catalyst systems. Processes for preparing high molecular weight polyalkenes of wide molecular weight distribution, by virtue of the particular catalyst system employed in the polymerization, are disclosed in British patent specification Nos. 1,154,884; 1,114,020; 1,170,299 and U.S. Pat. No. 3,509,117.
The aforementioned processes, which rely upon the nature of the catalyst system to produce polyalkenes of high molecular weight and wide molecular weight distribution are characterized by the disadvantages of slow polymerization rates. Slow polymerization rates produce low yields of polymer, relative to the catalyst residue and thus result in polymers which contain high amounts of catalyst residue contamination, which must be removed by washing. Large amounts of catalyst are necessary in these processes. In addition, the step of washing the resulting polymer free of catalysts render these processes uneconomical. Although the British patent specification No. 1,170,299 asserts that the process disclosed therein results in much higher yields which obviates the necessity of the washing step to free the polymer of catalysts, according to Example 1 of that reference, polymerization for seven hours in an absolute pressure of 5 atmospheres produces a polymer that contains 195 ppm of titanium. The titanium content of that product far exceeds the present requirements that the titanium content is less than 30 parts per million. Thus, because of the present requirements concerning titanium content in polyalkene polymers, the process according to British patent specification No. 1,170,299 would necessitate the step of washing polyalkene to free the polyalkene of catalyst residues. Additional processes have been disclosed in the prior art for producing .alpha.-alkenes of wide molecular weight distribution. It has been proposed to carry out .alpha.-alkene polymerizations in two or more stages to produce polyalkenes, particularly polyethylenes, of different molecular weights in the different stages and, hence, to produce a final product with a wide molecular weight distribution. Such processes are disclosed, for instance, in U.S. Pat. No. 3,074,922; German Auslegeschrift No. 1,138,940; French patent specification No. 1,236,365; and in U.S. Pat. Nos. 3,392,213 and 3,592,880.
According to the above U.S. Pat. No. 3,592,880 ethylene is polymerized in two stages: In one stage 5 to 30% by weight of the total amount of polymer is produced, while 70 to 95% by weight of the total amount of the polymer is produced in the second stage. In that stage in which 5 to 30% by weight of the total polymer is produced, 0 to 10% by volume of the gaseous phase is hydrogen, while in that second stage in which 70 to 95% by weight of the total polymer is produced, the hydrogen content is 20 to 80% by volume of the gaseous phase.
Luxemburg patent specification No. 43,556 discloses a process for polymerizing propylene to polypropylene characterized by a wide molecular weight distribution by controlling the amount of hydrogen used in the course of the polymerization, i.e. by varying the amounts of hydrogen used during the polymerization.
Although the prior art shows producing polyalkenes, particularly polyethylene, with wide molecular weight distribution by employing multistage polymerization processes, the resulting materials of these processes, when molded into articles and especially when blow-molded into bottles and into sheets of films, are characterized as containing non-homogeneities, which are attributed to gel formation within the polymer. Such non-homogeneities are manifested as small granular irregularities in the molded articles.
The cause of these irregularities has not been determined satisfactorily. It is assumed that the extreme values of the molecular weight of the polymers from the various reaction stages differ widely and that, in view of the molecular weight distribution of the polymer formed in each stage, slight amounts of material with very high molecular weight are formed which cannot, or with great difficulty can, be mixed homogeneously with the other polyalkene. However, this assumption has not been proved and is not to be considered a binding rationalization for the resulting in homogeneities in the aforementioned prior art polyalkenes.